The present invention relates generally to RAKE receivers for code division multiple access (CDMA) systems, and more particularly, to a method and apparatus for determining the finger placement of RAKE fingers in a RAKE receiver.
In a wireless communication system, a transmitted signal may travel through multiple propagation paths so that the received signal is a composite of multiple time-shifted versions of the signal. The different time-shifted versions of the received signal, referred to herein as signal images, suffer from different phase and attenuation effects. The multiple time-shifted signal images combine in an unpredictable manner resulting in signal fading.
CDMA receivers typically employ a RAKE receiver to combat signal fading due to multi-path propagation. The goal of the RAKE receiver is to detect the individual signal images and combine them coherently. A RAKE receiver typically includes a plurality of correlators, sometimes referred to as fingers, to separately despread different time-shifted signal images, and a combiner to combine the correlator outputs. For example, a RAKE receiver may detect and combine the M strongest signal images. A path searcher processes the received signal to identify the delays corresponding to the strongest signal images, and a finger placement processor determines the finger placement based on those delays. The process of finger placement comprises the assignment of a delay to each RAKE finger to align the RAKE finger in time with a signal image. A simple finger placement strategy is to assign the delays of the J strongest signal images found by the path searcher to respective RAKE fingers.
Typically, finger placement starts by generating an estimated power delay profile (PDP) over a defined search window that gives the signal power as a function of delay. The path searcher measures the signal power of the received signal samples. The spacing between samples defines a search grid and the signal power measurements define the PDP. One approach to finger placement, referred to herein as the “peak” approach, is to place fingers at or near the peaks or local maximas in the PDP. Ideally, the RAKE fingers would be placed at the exact delays corresponding to peaks in the PDP. Exact placement of the RAKE fingers at the peaks of the PDP is not always possible because the search grid does not always align with the peaks in the PDP. The actual peak in the PDP may fall between the grid points of the search grid.
In some circumstances, the path searcher may detect a dominant signal image in the received signal. This situation may occur, for example, when there is a direct line-of-sight path between the transmitter and the receiver, when there is a non-dispersive channel, or when one signal image is much stronger than other signal images. When a dominant signal image exists, the RAKE fingers are typically placed on a finger grid centered on the delay of the dominant signal image reported by the path searcher. Delay hypothesis testing can be used to determine the relative timing delay between the current finger placement and the dirac-like channel impulse response in order to track the path of the dominant signal image. Knowing this relative delay allows for interpolation of despread values at non-sample positions thus increasing the SINR at the output of the combiner compared to non-interpolating schemes.
Conventional methods for delay hypotheses testing suffer from a high computational complexity to achieve a given accuracy with respect to the relative delay estimate. The reason for the computational complexity is that the number of hypotheses to be tested depends on the uncertainty interval of the path searcher result, which is at least based on the sampling period distance and on the required delay resolution. Assuming typical values, i.e., a sampling period distance of ¼ chip duration and a required delay resolution of 1/32 chip to support 64-QAM, 9 different hypotheses needs to be tested.